1. Field of the Invention
The present invention relates to a frequency switching method for a Radio Data System (RDS). More particularly, the present invention relates to a method and an apparatus for switching from the main frequency to an Alternative Frequency providing the same station.
2. Description of the Related Art
An RDS is a system for providing information services (such as a traffic information service, a paging service, and an automatic tuning) using a subcarrier of a Frequency Modulation (FM) broadcast. A terminal, such as mobile terminal and a navigation device can be configured to provide a user with the aforementioned information service using RDS data.
The RDS data may include an Alternative Frequency (AF) list, a broadcast station name, a nationality of station, and a Program Identification (PI) code identifies the station.
Typically, an RDS-enabled terminal provides an automatic tuning function to switch from the main frequency to one of alternative frequencies listed in the AF list. This means that the user does not need to retune the frequency, but the terminal measures the received signal strength of the main frequency and switches, when the received signal strength is equal to or less than a threshold value, to an alternative frequency, resulting in improvement of user convenience.
However, when the terminal measures the received signal strength of the alternative frequency and determines the PI code, a sound discontinuation effect occurs. The sound discontinuation is an effect in which no sound is heard for a few milliseconds or seconds in the middle of listening to the radio.
Typically, the alternative frequency-switching function is configured to be executed in an environment where the electric field does not vary abruptly. In such an environment, there is no need to determine the received signal strength of the main frequency so frequently that the received signal strength measurement cycle can be set to a relatively long time.
In a case where the long measurement cycle is applied in the environment wherein the electric field varies abruptly, the terminal cannot switch to an alternative frequency strong enough in the received signal strength in due time.
FIG. 1 illustrates a broadcasting system in which stations are overlapped in their radio coverage regions according to the related art.
Referring to FIG. 1, three frequencies, i.e., 102.4 Mhz, 105.1 Mhz, and 88.3 Mhz, are overlapped in a region 11. In the region 11, the electric field varies abruptly such that the received signal strength of the main frequency is likely to decrease. At this time, the alternative frequency is likely to have a good signal strength and thus it is a good policy to switch to the alternative frequency quickly. For this purpose, the received signal strength determination cycle needs to be set to a short time.
However, if the received signal strength determination cycle is too short, the frequency switching occurs frequently, resulting in an increase of the number of sound discontinuations.
Especially in a building or basement, both the current and alternative frequencies become weak in strength. In this case, the frequency switching to the alternative frequency is likely to fail due to the weak signal strengths of both the current and alternative frequencies. If the terminal attempts frequency switching without consideration of such environment, the sound discontinuation effect occurs frequently without successful frequency switching.
The weak electric field environment may occur temporarily when the terminal passes through a tunnel or has change in antenna conditions. In such a case, since the electric field condition is recovered soon, maintaining the main frequency is preferred. However, the terminal of the related art attempts frequency switching repeatedly because the received signal strength of the current signal is poor without consideration that the poor signal condition can be recovered soon, resulting in frequency sound discontinuation effects.
There is therefore a need for an apparatus and a method for switching between frequencies in the RDS that is capable of reducing the occurrence of sound discontinuation effect in consideration of various radio listening environments.